Description of the Art
Direct radiographic imaging using detectors comprising a two dimensional array of tiny sensors to capture a radiation generated image is well known in the art. The radiation is imagewise modulated as it passes through an object having varying radiation absorption areas. Information representing an image is, typically, captured as a charge distribution stored in a plurality of charge storage capacitors in individual sensors arrayed in a two dimensional matrix.
X-ray images are decreased in contrast by X-rays scattered from objects being imaged. Anti-scatter grids have long been used (Gustov Bucky, U.S. Pat. No. 1,164,987 issued 1915) to absorb the scattered X-rays while passing the primary X-rays. A problem with using grid, however, is that whenever the X-ray detector resolution is comparable or higher than the spacing of the grid, an image artifact from the grid may be seen. Bucky recognized this problem which he solved by moving the anti-scatter grid to eliminate grid image artifacts by blurring the image of the anti-scatter grid (but not of the object, of course).
Improvements to the construction of anti-scatter grids have reduced the need to move the grid, thereby simplifying the apparatus and timing between the anti-scatter grid motion and X-ray generator. However, Moire pattern artifacts can be introduced when image capture is accomplished through the direct radiographic process or when film images are digitized. (The Essential Physics of Medical Imaging, Jerrold T Bushberg, J. Anthony Seibert, Edwin M. Leidholdt, Jr., and John M. Boone. c1994 Williams & Wilkins, Baltimore, pg. 162 ff.).
When the X-ray detector is composed of a two dimensional array of X-ray sensors, which generate a two dimensional array of picture elements, as opposed to film, the beat between the spatial frequency of the sensors and that of the anti-scatter grid gives rise to an interference pattern having a low spatial frequency, i.e. a Moire pattern.
There are two possible approaches to solving this problem. The first, described in U.S. Pat. No. 5,666,395 to Tsukamoto et al. teaches Moire pattern prevention with a static linear grid having a grid pitch that is an integer fraction of the sensor pitch.
In the case where the sensors are separated by dead spaces, i.e. interstitial spaces which are insensitive to radiation detection, Tsukamoto teaches to make the grid pitch to correspond to the sensor pitch and to hold in a steady positional relation to the detector such that the grid elements are substantially centered over the interstitial spaces.
A problem with the above proposed solution, which uses a static grid, is that it is often impractical to position and to maintain the anti-scatter grid in a desired fixed position relative to the radiation detector array.
A second approach, originally proposed by Bucky in U.S. Pat. No. 1,164,987 proposes moving the anti-scatter grid during radiation exposure to blur the artifact images generated by the grid.
The use of a moving grid appears a reasonable solution but for one problem. In modem radiographic equipment the exposure time is determined by automated exposure control devices. The total exposure time is, therefore unknown, and as a consequence the bucky must be maintained in motion for an undetermined length of time, at least long enough for the longest anticipated exposure. Using a single stroke unidirectional linear velocity profile is impractical because as the exposure becomes longer the size of the bucky and the length of the bucky path become far too large to be accommodated in a useful package. The solution adopted by the art is to provide an oscillating bucky which can be continuously on for so long as the exposure lasts.
While this is an ingenious solution it also presents certain practical problems, particularly related to the direction change in the bucky movement at the two path ends where the grid movement becomes zero prior to reversing direction. A number of patents have issued describing different arrangements to solve this reversal problem including oscillating the grid with a velocity that increases as the grid approaches the travel limits prior to reversal of the travel direction, or controlling the location of the grid interstitial spaces at the reversal point to avoid creation of artifacts.
With the exception of the solution proposed by Tsukamoto et al., the above methods have been proposed to solve the problem of a film grid combination rather than direct radiographic imaging application and as such are primarily concerned with the elimination of shadow type artifacts rather than the Moire patterns which are generated when using a direct radiographic detector comprising rows and columns of individual image detecting sensors with an anti-scatter grid. Direct radiography is a relatively new technology and often requires new and different solutions better fitted to the new set of problems associated with it. The art originally started with a grid which was moveable in one direction. When this approach failed, due to innovations in the radiation exposure equipment, the art solved the new problems by inventing the oscillating grid. This solution worked for radiographic film exposure, but does not adequately solve the Moire type problems associated with direct radiography detectors. There is still a need in the art for a single stroke radiation anti-scatter device suitable for a wide range of exposure windows, and tailored to reduce Moire-pattern artifacts in digital radiograms.